1. Field of the Invention
The present invention generally relates to a paging radio communication system and method, and more particularly to a private paging system in which a selective calling generated by a call controller is transmitted to a pager (a paging radio receiver or station) via radio transmitters.
2. Description of the Prior Art
In a paging radio communications system, such as a private paging system, a selective calling signal generated by a call controller is transmitted via radio transmitters. Normally, one radio transmitter covers a narrow service area. In this point of view, a plurality of radio transmitters spaced apart from each other are used so that a wide service area can be obtained. In order to prevent an area in which radio waves from the transmitters are not received at all, the radio transmitters are located so that boundary areas of adjacent service areas overlap each other. However, there is a possibility that radio waves having different phases may be received in the overlapping boundary areas. In this case, the radio waves interfere with each other, and cannot be correctly received by radio receivers (pagers). This decreases the receiving rate of the radio receivers.
In order to eliminate the above problem, the following method has been proposed. Each of the radio transmitters executes a carrier sensing operation before signal transmission, and determines whether or not at least one of the other transmitters is emitting a radio wave. When the above determination is affirmative, each of the radio transmitters delays the transmission timing in order to prevent two or more radio transmitters from concurrently emitting radio waves. The above method is described in the following document: "PAGING RADIO EQUIPMENT FOR SPECIFIED LOW POWER RADIO STATION, RCR STANDARD", Research & Development Center for Radio Systems, RCR STD-19, Jan. 25, 1990, the disclosure of which is hereby incorporated by reference.
FIG. 1 is a block diagram of a conventional paging radio communications system which conforms to the RCR STD-19 standard. The conventional paging radio communications system shown in FIG. 1 comprises a call controller 3, and radio transmitters 4.sub.1 -4.sub.3. The call controller 3 generates a selective calling signal (which is referred to as a POCSAG signal in the RCR STD-19 standard) in response to a request from an exchange, such as a private branch exchange (not shown). The selective calling signal has information indicating an identifier of a called station. The call controller 3 sends the selective calling signal to the radio transmitters 4.sub.1 -4.sub.3. Each of the radio transmitters 4.sub.1 -4.sub.3 comprises a memory 41, and a controller 42. The selective calling signal from the call controller 3 is temporarily stored in the memory 41 of each of the radio transmitters 4.sub.1 -4.sub.3 under the control of the respective controllers 42. Then, the identical selective calling signals are respectively read from the memories 41 of the radio transmitters 4.sub.1 -4.sub.3 at respective transmission timings in accordance with a procedure prescribed in the RCR STD-19 standard, and are then transmitted in the form of radio waves.
FIG. 2 is a flowchart of the procedure prescribed in the RCR STD-19 standard. The radio transmitters 4.sub.1 -4.sub.3 wait for receipt of the selective calling signal in step S1. The radio transmitters 4.sub.1 -4.sub.3 receive the selective calling signal, and start a calling process in step S2. In the calling process, the radio transmitters 4.sub.1 -4.sub.3 carry out a carrier sense operation in step S3 in order to determine whether or not there is a radio wave being emitted from one of the other radio transmitters and having a frequency identical to the frequency to be used (selected). In response to receipt of the selective calling signal from the call controller 3, the controller 42 of each of the radio transmitters 4.sub.1 -4.sub.3 selects one of eight predetermined transmission delay times and delays transmission of the selective calling signal in step S5. For example, the eight predetermined transmission delay times are equal to 600, 650, 700, 750, 800, 850, 900 and 950 ms. When the respective transmission delay times of the radio transmitters 4.sub.1 -4.sub.3 have elapsed, the respective controllers 42 determine, in step S4, whether or not radio waves from other radio transmitters are sensed. When the controllers 42 of the radio transmitters 4.sub.1 -4.sub.3 respectively determine that no radio waves are sensed, the radio transmitters 4.sub.1 -4.sub.3 transmit the identical selective calling signals corresponding to the selective calling signal and having frequencies identical to each other in step S6.
According to the RCR STD-19 standard, the selective calling signal may be in either analog or digital form, and a transmission rising time and a transmission falling time are respectively equal to or shorter than 0.02 seconds. Further, according to the standard, the allowable continuous transmission time is equal to or shorter than 15 seconds in analog form, and equal to or shorter than 5 seconds in digital form. However, the RCR STD-19 standard does not prescribe the format of the selective calling signal.
In the case where a radio wave from one of the other radio transmitters is sensed when the selected random transmission delay time has elapsed, one of the random transmission delay times is selected again. Transmission is delayed until the newly selected random transmission delay time elapses. When the newly selected random transmission delay time has elapsed, the carrier sense operation is executed again. In this manner, the above operation is repeatedly carried out until transmission of the selective calling signal becomes successful.
The radio receivers wait for receipt of the selective calling signal in step S9. When a radio receiver receives the selective calling signal addressed thereto, it transmits a response signal in step S10.
According to the RCR STD-19 standard, it is necessary to provide a transmission rest time equal to, for example, one second, after transmission of the selective calling signal (step S7). Then, the radio transmitters 4.sub.1 -4.sub.3 respectively wait for receipt of the selective calling signal from the call controller 3.
As described above, in order to transmit the selective calling signals from the radio transmitters 4.sub.1 -4.sub.3 in accordance with the RCR STD-19 standard, it is necessary to temporarily store the selective calling signal from the call controller 3 in the memories 41 of the radio transmitters 4.sub.1 -4.sub.3 and independently transmit the selective calling signals at respective transmission timings.
Referring to FIG. 3, when each of two radio transmitters is capable of sensing a radio wave emitted from the other radio transmitter, the carrier sense operation is correctly carried out, and prevents the radio transmitters from concurrently transmitting radio waves respectively having identical frequencies according to the RCR STD-19 standard. Hence, a radio receiver located in an overlapping area E does not concurrently receive the radio waves from the radio transmitters. Hence, a decrease in the receiving rate does not occur.
FIG. 4 shows a case where each of the radio transmitters cannot sense a radio wave emitted from the other radio transmitter because of a barrier 5, such as a building, while the overlapping area E is obtained. In this case, each of the radio transmitters does not sense the radio wave from the other radio transmitter, and hence transmit the selective calling signals independently of each other according to the RCR STD-19 standard. The radio transmitters send the selective calling signals at the respective transmission timings, and hence the selective calling signals are not in phase with each other. The selective calling signals interfere with each other, and a radio receiver in the overlapping area E fails to receive the selective calling signal.